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30                                        OXALIC ACID

and of variable specific gravity but usually 1-4 or 1-5-1-52, containing 65%
or 94-99-5% HNOjj; as fuming acid, which is reddish-yellow or reddish-
brown and emits dense reddish fumes in the air, D = 1-48-1-52 (86-99%
HN03) and is a mixture of nitric acid with nitrogen tetroxide and nitrous
acid (the latter in small quantity, especially in the very dark filming acid).
The usual impurities to be sought for in nitric add are : sulphuric and
hydrochloric acids, heavy, earthy and alkali metals, iodine and its com-
pounds and nitrous compounds.

1.  Sulphuric Acid.—10 c.c. are evaporated on a steam-bath to i c.c.,
then diluted with 10 c.c. of water and tested with barium nitrate :   no
turbidity should appear.

2.  Hydrochloric Acid, -r c.c., diluted with 5 c.c, of water, should
give no turbidity with silver nitrate.

3.  Metals.—Diluted in  the  ratio   i : 2  and  rendered  alkaline  with
ammonia, the acid should not be changed by ammonium sulphide or oxalate.

i c.c., diluted with 10 c.c. of water, should give no immediate colora-
tion with potassium ferrocyanide (iron,).

50 c.c., evaporated on a steam-bat)i, should give no appreciable

4.  Iodine (loclic Acid).—i c.c., diluted with 2 c.c. of water and shaken
with a few drops of chloroform, should give no coloration to the latter,
even after addition of a fragment of xinc.

5.  Nitrous Compounds.   -To 5 c.c. of the acid, diluted with 5 volumes
of water, arc added a few drops of nornud potassium permanganate solution,
which is decolorised if nitrous compounds are present.

For the quantitative determination of nitrons compounds in the fuming
acid, Lunge and Marchlewski's method may be followed : From a well-
calibrated, narrow burette, divided into .,',, e.e. so that o-or c.c. can lie
readily measured, the acid is allowed to fail drop by drop into a, measured
volume of seminormal permanganate solution (15*^03 grains KMn()4 per
litre) kept at 40", until dceolorisation occurs. Before, the titration the
acid is left for some time in the burette to assume the air-temperature,
which is measured with an accurate thermometer. The number of e.c. of acid
taken, multiplied by its specific gravity determined at the temperature of
titration, gives the weight of acid used in the titration. The nitrous com-
pounds, calculated as Na04) are, expressed pur 100 parts of the acid. The
titre of the permanganate is determined by means of iron in the ordinary
way. i c.c. N/3-permanganatc -.0-023 gnun Nj.(.)4.

In acid for technical purposes the presence of nitrous products is allowed
—in that used in dynamite factories to the extent, of 2% ((hittmann).

H,CaQ4 + 2'Ha() *-->: 126-05 (126)

Colourless crystals, soluble in about 10 parts of cold water or 2-5 of
boiling water, and in alcohol. Its most frequent impurities art; sulphates,
chlorides, ammonia, alkalies and calcium, and sometimes small quantities
of copper, lead and iron.nd earthy metals and organic matter (derived especially from the vessels).en the two weights the acetic acid is calculated.                        -jl ,